Question

What the difference between A a1(5); and A a2 = A(5) ? Both of the works, but I really want know the difference between them, because I used method 2 in one of my project and I suffered from a bug which is fixed after I change to method 1. Thanks in advance!

class A {
public:
    int val;
    A() : val(0) {}
    A(int newVal) : val(newVal) {}
};

int main()
{
    A a1(5);  // method 1
    A a2 = A(5); // method 2
}

Answer 1

A a1(5);  // method 1
A a2 = A(5); // method 2

The first one is called direct initialization, the second one is called copy-initialization.

The second one will NOT compile if you make the copy-constructor inaccessible or/and don’t define it as:

class A {
public:
    int val;
    A() : val(0) {}
    A(int newVal) : val(newVal) {}
private:
    A(A const&);  //the second one will not compile
};

Now the second one will not compile. Note that it will not compile in both cases:

• If the copy-constructor is defined, but is inaccessible (either it is private or protected).
• If the copy-constructor is declared, but not defined.